This invention relates to an exhaust control system for use in an internal combustion engine for controlling the flow of exhaust gases discharged from the engine to the atmosphere in accordance with engine operating conditions.
Exhaust gases are discharged from an engine to the atmosphere through an exhaust system which includes a catalytic converter, a pre-muffler and a main muffler placed in an exhaust passage extending from the exhaust manifold of the engine. The exhaust passage has an effective area through which exhaust gases are discharged to the atmosphere. The exhaust passage effective area is determined in view of such factors as desired engine exhaust pressure and low-frequency pulsation sound arresting effect. The greater the exhaust passage effective are, the smaller the engine exhaust pressure, whereas the smaller the exhaust passage effective area, the greater the sound arresting effect. In order to provide adequate engine exhaust pressure, the exhaust passage effective area must be relatively large. However, a large exhaust passage effective area results in an excessive drop of pulsation sound arresting effect at low engine speeds. In addition, an attempt to ensure sufficient pulsation sound arresting effect by decreasing the exhaust passage effective area will cause an excessive increase of engine exhaust pressure which results in an excessive engine output reduction at high engine speeds.
This delemma can be overcome by using a control valve to change the effective area of the exhaust passage in accordance with engine speed, as disclosed, for example, in Japanese Utility Model Kokai Nos. 59-102922 and 59-156104. The control valve remains closed to restrict a part of exhaust gas flow through the exhaust passage at low engine speeds and opens to increase the effective area of the exhaust passage at high engine speeds so as to ensure both sufficient pulsation sound arresting effect at low engine speeds and sufficient exhaust pressure reduction at high engine speeds.
With such apparatus which include a control valve operable to change the effective area of the exhaust passage in accordance with engine speed, however, the columnar resonance will increase in the exhaust system, causing a high level alternant due to emission of impact waves to the exterior during deceleration while the control valve remains open to widen the exhaust passage effective area. This is true particularly when rapid deceleration is resumed after rapid acceleration.
Therefore, the present invention provides an engine exhaust control system which is free from the above described disadvantages.